The present invention relates to a high-speed digital signal processor and more particularly of the type of processor utilizing a look-up table to generate the output signal from an input signal such as a video signal.
In the prior art, it is known to use a read-only memory (ROM) for processing digital signals at high speed. As illustrated in FIG. 1, digital signals, such as a video signal, are applied in parallel to 8-bit input terminal 10, and from there to the 8-bit address input 12 of ROM 14. The digital video signals may come for example from an A/D converter (not shown) operated at a clock rate. The input of the converter would have an analog video signal applied to it so that the input video signal is sampled and then 8-bit quantized (256 grey levels) at the clock rate. The clock which drives the ADC (not shown) is also applied to the read input 16 of ROM 14. For each possible signal value applied to the ROM address, there is a corresponding ROM memory location which contains data which may be read from 8-bit output 18 on to an 8-bit output terminal 20. Typically, line 20 will be 8-bits wide for television applications, but may have other values. Thus, depending upon the data stored in the ROM, the signal may be processed. For example, if the data stored in each ROM address is a value representative of half the address value, the data read out will represent one-half of the input, and therefore ROM 14 functions as a 2:1 attenuator. Other amplitude functions are possible. For example, in order to provide limiting, each memory location may have, as before, stored data representing half the address for the memory location. However, memory in the addresses above a particular value may all contain stored data representing half the address of the limiting value.
This will give a linear gain of one-half up to the limiting value and no increase after that value. In a similar manner, pedestal functions may be generating by having at each memory address data representing the address value summed with a fixed offset value. Gamma correction can be accomplished by setting the data stored in each memory location in accordance with a predetermined exponential gamma function such as a square-root function.
The described prior art system has the disadvantage that the transfer function is fixed by the ROM memory. If such a ROM were used for example to control the gain, pedestal and gamma of one of three color representative signals derived from a vidicon, analog controls would have to precede the ADC so as to standardize the signal levels entering the ROM for further processing. In the absence of such a standardization, the ROM could not be used because the transfer function could not be changed to meet varying conditions. It would be possible, of course, to change the ROM as described in U.S. patent application filed July 17, 1980, Ser. No. 169,680, for Terence Smith and Frank Marlowe and entitled "SYNCHRONIZING CIRCUIT ADAPTABLE FOR VARIOUS TV STANDARDS". While this allows changing ROMs when a different use is intended, it does not solve the problem of variations encountered during use.
It is therefore desirable to have a signal processor that can change its transfer function during use.